"I’ve worked in wildlife disease all my life, and I thought I’d seen some pretty grim things," Richard Kock, of the Royal Veterinary College in London, told the Times' Carl Zimmer. "But this takes the biscuit."

Now, in a press release published by the Saiga Conservation Alliance (which we first saw at IFLScience), scientists have identified the cause of the tragedy: a microbe called Pasteurella multocida.

More than 90% of saiga live as a subspecies in the grassy steppes of Kazakhstan and parts of Russia. Another subspecies lives in Mongolia.

Adult males grow sharp horns and weigh up to 110 pounds, while females have no horns and weigh up to 90 pounds.

Saiga travel in herds of 30 to 40 animals, yet are known to migrate in clusters of tens of thousands of individuals. The World Wildlife Fund says these super-herds are among "the most spectacular migrations in the world."

Conservationists banded together in 1994 to do something about the problem — and they did, helping grow the Betpak-dala population to about 257,000 animals by April 2014.

But then, in May of 2015, outbreaks of a mysterious, deadly disease wiped out more than half of the remaining saiga population worldwide. About 211,000 of them died last spring, including more than 88% of the Betpak-dala population.

Scientists have narrowed down the possible cause of the infection to one species of bacteria in the animals' stomach that turned deadly.

When scientists studied the dead animals, they found internal bleeding. Blood tests showed that the saigas suffered massive infections by bacteria called Pasteurella multocida. (Clostridium perfringens was also found but later ruled out as the cause.)

The normally harmless infections took hold because wild swings in temperature weakened the animals' immune systems. Warmer temperatures across the region also prompted the normally harmless bacteria to create dangerous toxins.

Mothers and their calves were most susceptible. "When symptoms appeared, death was only a few hours away. The herds showed up to 100% mortality, leaving only a few groups of animals alive," said conservationist Steffen Zuther.

Unfortunately, there's not much that can be done right now. "The likely stress caused by attempting to vaccinate them ... may be as likely to lead to mortality as the disease itself," Kock said in a release.